Inter-galactic Shock Acceleration and the Cosmic Gamma-ray Background

Abstract

We investigate numerically the contribution to the cosmic gamma-ray background from cosmic-rays ions and electrons accelerated at intergalactic shocks associated with cosmological structure formation. We show that the kinetic energy of accretion flows in the low-red-shift inter-galactic medium is thermalized primarily through moderately strong shocks, which allow for an efficient conversion of shock ram pressure into cosmic-ray pressure. Cosmic-rays accelerated at these shocks produce a diffuse gamma-ray flux which is dominated by inverse Compton emission from electrons scattering off cosmic microwave background photons. Decay of neutral pi-mesons generated in p-p inelastic collisions of the ionic cosmic-ray component with the thermal gas contribute about 30% of the computed emission. Based on experimental upper limits on the photon flux above 100 MeV from nearby clusters we constrain the efficiency of conversion of shock ram pressure into relativistic CR electrons to < 1%. Thus, we find that cosmic-rays of cosmological origin can generate an overall significant fraction of order 20% and no more than 30% of the measured gamma-ray background.